Refactor ESLint configuration and update test rules for clarity and consistency

- Consolidated and modularized `eslint.config.mjs` with defined rules for source, test, E2E, and scripts.
- Improved test and E2E rules with relaxed settings for flexibility and enhanced mocking.
- Standardized variable naming and removed redundant imports in unit and E2E tests.
- Updated error handling and comments to align with modern TypeScript best practices (e.g., `@ts-expect-error`).

frontend/e2e/admin-access.spec.ts

@@ -7,7 +7,6 @@ import { test, expect } from '@playwright/test';
 import {
   setupAuthenticatedMocks,
   setupSuperuserMocks,
-  loginViaUI,
 } from './helpers/auth';
 
 test.describe('Admin Access Control', () => {